1. Buy a device, https://wiki.postmarketos.org/wiki/Devices#Main . I suggest the PinePhone if you want something with decent power management in the next few months, or PinePhone Pro if you're willing to wait a few months for decent power management.
2. Find good programs on https://linmobapps.frama.io/
3. Figure out what functionality you're missing. Contribute improvements.
Keep in touch with the community at https://linmob.net/